1. Field of the Invention
The present invention relates to a zoom lens system mainly used in an electronic still camera (digital camera) having a zoom ratio more than 4 and including a wide angle range (half angle-of-view of more than 30°).
2. Description of the Prior Art
In recent years, there has been an increasing need for further miniaturization and higher definition in digital cameras; and the pixels of CCD imaging devices are further miniaturized. Accordingly, photographic lens systems of such digital cameras are required to have high resolution. Furthermore, a long back focal distance is also required in order to accommodate a filter group. In addition, an optical system for a color CCD is required to have good ‘telecentricity’ in order to prevent shading and color shift, i.e., the light rays which exit from the final lens surface and are incident on the imaging surface are to be as perpendicular to the imaging surface as possible.
As a miniaturized zoom lens system for a compact digital camera, a front-negative lens system (negative-lead type lens system) can be used in the case where a zoom ratio is up to around 3 to 4. Such lens systems can provide a wider viewing angle at the short focal length extremity, and can achieve further miniaturization of the lens system. In particular, the lens diameter of the first lens group (the most object-side lens element) can be made smaller. Due to the above features, the negative-lead type lens system is suitable for a ‘telescoping’ zoom lens system in which the distance between the lens groups is made shorter in an accommodating position. However, in such a zoom lens system, it is difficult to achieve a zoom ratio of more than 4.
A zoom lens system disclosed in U.S. Pat. No. 5,100,223 can be taken as an example of a zoom lens system having a zoom ratio of more than 4. In this zoom lens system, a zoom ratio of about 6 is achieved; however, the lens diameter of the front lens group is large, and miniaturization thereof is insufficient. Furthermore, the half angle-of-view at the short focal length extremity is about 25°. This means that the focal length at the short focal length extremity is sufficiently short thereat.
As another example, a zoom lens system disclosed in U.S. Pat. No. 5,570,233 achieves a wide angle-of-view, i.e., a sufficient short focal length at the short focal length extremity; however, the diameter of the front lens group is large, and a large number of lens elements are required. Accordingly, this zoom lens system is not appropriate for a camera having a telescoping zoom lens system.
In a zoom lens system for a compact telescoping-type zoom lens camera, thickness of each lens group is required to be smaller, in addition to a smaller lens diameter of the front lens group and a shorter overall length of the lens system. Generally, if an attempt is made to reduce the number of lens elements in order to reduce the thickness of the lens groups, the correcting of aberrations, on the other hand, becomes difficult. In order to adequately correct aberrations over the entire zooming range while further miniaturization is achieved, an appropriate refractive power distribution over each lens group and an appropriate lens arrangement are necessary.